Wireless networks have experienced increased development in the past decade. One of the most rapidly developing areas is mobile ad-hoc networks (MANETs). Physically, a MANET includes a number of geographically distributed, potentially mobile nodes sharing one or more common radio channels. Compared with other types of networks, such as cellular networks or satellite networks, the most distinctive feature of MANETS is the lack of any fixed infrastructure. The network is formed of mobile (and potentially stationary) nodes, and is created on the fly as the nodes communicate with each other. The network does not depend on a particular node and dynamically adjusts as some nodes join or others leave the network.
As wireless communication increasingly permeates everyday life, new applications for MANETs will continue to emerge and become an important factor in wireless communications. Yet, MANETs pose serious challenges to designers. Due to the lack of a fixed infrastructure, nodes may have to self-organize and reconfigure as they move, join or leave the network. Further, there may be no natural hierarchy or central controller in the network. Thus, many functions which are centralized in other types of networks have to be distributed among nodes in a MANET.
Furthermore, mobile nodes are often powered by batteries and have limited communication and computation capabilities. In addition, the distance between nodes will often exceed the nodes' radio transmission range, and transmissions may thus have to be relayed by other nodes before reaching their destination. Consequently, a MANET network typically has a multi-hop topology, and this topology changes as the nodes move around.
The MANET working group of the Internet Engineering Task Force (IETF) has been actively evaluating and standardizing routing protocols, including multicasting protocols. Because the network topology changes arbitrarily as the nodes move, information is subject to becoming obsolete, and different nodes often have different views of the network, both in time (i.e., information may be outdated at some nodes but current at others) and in space (i.e., a node may only know the network topology in its neighborhood and not far away from itself).
MANET routing protocols thus need to adapt to frequent topology changes, possibly with less than accurate information. Because of these unique requirements, routing in MANETs is very different than in other networks. Gathering fresh information about the entire network is often costly and impractical. Thus, some routing protocols are reactive (i.e., on-demand) protocols. That is, they collect routing information only when necessary and only to destinations to which they need routes, and do not maintain unused routes. In this way the routing overhead may be reduced compared to pro-active protocols, which maintain optimal routes to all destinations at all times. Ad Hoc on Demand Distance Vector (AODV), Dynamic Source Routing (DSR) and Temporally Ordered Routing Algorithm (TORA) are examples of reactive routing protocols presented at the MANET working group.
An example of a proactive routing protocol is found in Clausen et al. entitled “Optimized Link State Routing Protocol,” IETF MANET Working Group, Internet Draft, October 2003. Examples of other various routing protocols include Destination Sequenced Distance-Vector (DSDV) routing, which is disclosed in U.S. Pat. No. 5,412,654 to Perkins, and the Zone Routing Protocol (ZRP), which is disclosed in U.S. Pat. No. 6,304,556 to Haas. ZRP is a hybrid protocol using both proactive and reactive approaches.
In command/control MANET applications where messages are relayed back and forth to a command/control platform or node, an imbalance in the workload typically results. This is because nodes closer to the command/control platform have the added burden of routing messages between outlying or downstream nodes and the command/control platform, as well as handling their own messages. During a network application broadcast that requires a respective reply from each mobile node in the network, this burden may become excessive for nodes close to the command/control platform. As a result, significant battery drain and bandwidth usage may result.
One approach to address this problem is set forth in an article by Kunito et al. entitled “An Ad-hoc Routing Control Method in Sensor Networks.” This article proposes a new MANET routing protocol called Integrated Source Routing in Ad-hoc Networks with Adaptive Construction (ISAAC). The purpose of the proposed protocol is to reduce the number of control packets that are transmitted during route discovery, particularly over the links nearest to the requesting or source node. To do so, routing response control packets are “integrated” as they propagate back toward the source node and are sent together along the next hop to the source node. To integrate the response packets, each intermediate node waits a time t, and at the end of this time the intermediate node forwards the information it has collected from other nodes during this period. The time that an intermediate node waits is based in part upon a maximum hop count of the route from source to destination node, and the hop count between the source node and the intermediate node.
One potential drawback of such an approach is that each intermediate node has to wait long enough to allow outlying nodes a chance to receive the control packet and generate a response thereto. However, response times may vary significantly based upon network usage and other factors. Thus, to be effective the intermediate node would need to wait for the maximum expected time it could take outlying nodes to respond assuming worse-case response delays. Yet, imposing such response delays may significantly increase the time required to complete a broadcast message reply sequence, which may not be desirable in certain applications.